With the recent increase in portability of communication devices, it has been desirable to provide communications in different frequency bands. Such an arrangement permits communications in different locations around the world in which one or more of the different bands are used, provides a backup so that the same information can be provided at the different bands, or permits different types of information to be provided to the device at the different frequencies.
In many instances, for example due to space/design considerations, it is desirable to limit the number of separate antennas to a single combined structure that functions in the multiple bands. One particularly useful combination of bands includes very high frequency (VHF) band (about 136-174 MHz) and the global positioning satellite (GPS) band (about 1575 MHz, 10 times higher than the VHF band). This combination is particularly desirable for public safety providers (e.g., police, fire department, emergency medical responders, and military) who have used the VHF band maintained exclusively for public safety purposes. With the advent of GPS, it has become desirable to be able to determine locations of the public safety providers to better manage increasingly scarce resources, coordinate quicker response, and guide personnel safely through potentially dangerous situations.
It is especially challenging however to combine individual antennas with these bandwidths into a single structure. To be an effective radiator, antennas (also called radiating elements) have electrical lengths of λ/4. Thus, a VHF radiating element has a relatively long electrical length of λ/4 at the center of the VHF band, or about 50 cm, while the GPS radiating element of λ/4 is about 5 cm.
Unlike the VHF radiating element, the peak gain of the GPS radiating element is directed upward (away from feed point or the base of the radiating element) toward the GPS satellites. Unfortunately, the upward pointing antenna peak gain of GPS radiating elements of length λ/4 is relatively low in antenna structures combining VHF and GPS radiating elements. Simulations have shown that it would be desirable to extend the length of the GPS radiating element to 3λ/4 at the center of the GPS band to increase this gain and improve the upward radiation pattern. However, increasing this length to 3λ/4 detrimentally affects the performance in both bands when implemented in certain structures. Specifically, in these structures, the GPS radiating element consumes the majority of the current when attempting to excite the VHF radiating element, thereby suppressing the gain of the VHF radiating element. Further, in some of these certain structures, exciting the GPS radiating element instead excites the VHF radiating element, decreasing the gain of the GPS radiating element.
Accordingly, it is desirable to provide a combined antenna structure that has sufficient peak gain for multiple frequency bands while retaining a relatively small form factor.